1. The Field of the Invention
The present invention relates generally to optical sensors. More specifically, the present invention relates to reflective optical sensors for sensing the presence and absence of an object.
2. The Relevant Technology
Optical sensors are used in a wide variety of applications. One application where optical sensors are of particular use is to detect the presence and absence of an object. Two types of optical sensors that detect the presence and absence of an object are interrupt optical sensors and reflective optical sensors.
Interrupt optical sensors include an optical emitter and an optical receiver located on opposite sides of an object to be sensed. The presence of the object is indicated when the object breaks the optical link between the optical emitter and the optical receiver. The absence of the object is indicated when the optical transmission of the optical receiver is received by the optical emitter.
Reflective optical sensors can include an optical emitter and an optical receiver located on the same side of an object to be sensed. The presence of the object is indicated when an optical emission from the optical emitter is reflected from the surface of the object and is received by the optical receiver.
Reflective optical sensors can present several advantages over interrupt optical sensors. Reflective optical sensors are advantageous because both the optical transmitter and the optical receiver are located in a single package. A single package can eliminate additional screws, brackets, packaging, and shipping costs related to a two package interrupt sensor design. In addition, a reflective optical sensor may be relatively small in size, and more adaptable to a greater variety of placements because it is in a single package. The optical transmitter and receiver of the reflective optical sensor is also typically aligned during manufacturing, rather than on-site by the consumer often preventing misalignment by the consumer.
Conventional reflective optical sensors typically include a light emitting diode (“LED”) and an optical receiver (e.g. a photodiode or a phototransistor) mounted within side-by-side cavities in a sensor housing. The cavities must be of a sufficient depth to prevent undesirable cross talk between the LED and the optical receiver when an object is not present. Although this typical reflective optical sensor has been implemented in a number applications, it nonetheless presents a number of limitations.
Conventional reflective optical sensors using a LED emitter are limited in the concentration of light emitted from the LED resulting in a relatively short distance at which they can sense an object. LEDs also have a broad angular emission window projected in a large undefined angular range similar to a conventional light bulb. In certain applications, this results in low intensity of reflected light, excessive cross talk, lower resolution, and less accurate detection of the presence and absence of an object.
In addition, a conventional reflective optical receiver using typical photodiodes and photo transistors may have a low contrast ratio that may not be as sensitive to changes in illumination where fairly small changes in illumination are expected. Typical photodiodes and photo transistors may also detect certain low levels of light interference falsely indicating the presence of an object. In addition, typical photodiodes and photo transistors can operate in a substantially linear relationship between incident light intensity and photocurrent produced. As a result, typical photodiodes and photo transistors may produce interference current signals at low light levels supplying a less accurate and potentially false indication of the presence of an object.